In the exploration for hydrocarbons, it is highly desirable to precisely identify earth formation features at the various borehole depths. Many of these features have a fine structure which often can only be determined from the investigation of a core of the borehole as it is drilled. Coring, however, is an expensive time consuming process and in itself may at times alter the cored specimen so as to degrade the reliability of the interpretations of the features of the earth formation from which the core specimen was drawn.
Some of the features that are highly desirable to identify are fine beddings and facies, the heterogeneity of carbonate deposits and the structure of fractures. The detection of beddings, for example, includes detecting shaly-sand sequences where the shales establish a basal contact for each sequence. Facies identification involves identifying the lithology between basal contacts, a type of partitioning of a well log into regions that can be analyzed in greater detail. The analysis of carbonates involves detecting non-homogenous features such as are due to irregular cementation, variations in the pore sizes, small scale lithology changes, etc. Fractures play a major role in the flow characteristics of reservoir rock. Therefore, the measuring or detecting of fractures, determining their orientations, density, height, vertical and lateral continuity is highly desirable.
In a copending patent application entitled "Method And Apparatus For Electrically Investigating A Borehole" filed by Gianzero et al on July 30, 1981, bearing Ser. No. 288,554, now U.S. Pat. No. 4,168,623 and assigned to the same assignee as of this invention, an earth formation investigating tool is described with which borehole wall features of the order of millimeters in size can be detected. The tool includes an array of small crossection survey electrodes (buttons) which are pressed towards the borehole wall and each button injects an electric current into the adjoining formation. The individual button currents are monitored and signals representative of button currents are recorded as curves as a function of depth. The measured button currents reflect the resistivity of the material in front of each button. In order to achieve a high resolution investigation, the electrodes are arranged in an array of multiple rows. The electrodes are so placed at intervals along a circumferential direction about the borehole axis as to inject survey currents into borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole wall. In this manner a detailed high resolution resistivity or conductivity investigation of the borehole wall can be made.
A high resolution investigation of a continuous segment of an earth formation around a borehole may be done with measuring devices that are vertically spaced from each other on the investigating tool, but whose respective measurements at a common depth are to be depth correlated by depth shifting. In such case the depth shifting need only be by an amount that is a function of the velocity of the tool. Such velocity is typically measured by monitoring the velocity of the cable from which the tool is suspended. The motion of the tool itself, however, is not always equal to the cable velocity since the tool often sticks then slips and rapidly moves ahead as cable tension increases or the tool oscillates up and down, much like a yo-yo, at the end of a long flexible cable. Hence, simple monitoring of cable velocity at the surface is not a precise measurement of tool motion at any one instant of time so that depth shifting of high resolution measurements often involves inaccuracies attributable to non-uniform tool velocity.
Techniques have been proposed to determine tool velocity so as to be able to make a correct correlation of the measured parameters. One such technique as employed in a dipmeter, which is a bedding angle detector, involves a pair of electrodes which are vertically spaced from each other by a known small distance. Survey currents injected by these electrodes should be the same except for a small constant displacement. Where such displacement is not constant, the tool velocity is known to vary. A good description of such technique is found in an article entitled "The High Resolution Dipmeter Tool" by L. A. Alland and J. Ringot and published in The Log Analyst of May-June 1969.
As described in the latter article, the measurement of the velocity of the tool may be obtained by correlating the survey currents from the electrodes which are vertically spaced from each other by a known small distance. The correlation may then yield a measurement of the actual tool velocity so that the true distance by which data must be depth shifted for proper depth correlation can be determined. Such technique, however, requires continuous correlation computations over a substantial interval for proper speed correction and is subject to errors because of a smearing effect of the correlation and when the survey currents from the electrodes do not correlate well such as during a stick and slip condition of the tool.
In another technique for determining the actual speed of the tool, accelerometers are used which provide precise measurement of tool velocity if the integration of accelerometer data does not include significant errors. As a practical matter, however, the accelerometer data alone is not sufficiently accurate to enable the accurate depth shifting of high resolution data from a high resolution investigation of a borehole wall.
Although the high resolution investigation with a tool as described in the aforementioned Gianzero application yields significant information about the borehole wall, it becomes quite cumbersome to display such information in the conventional wiggle trace format. For example, as illustrated with reference to FIG. 2 herein, the large number of electrode buttons employed in an investigation yield in the aggregate a large number of traces that are difficult to analyze.